1. Apple may not have an existing partnership or agreement with Sina Mail, hence they cannot invite or integrate it into their services.
2. Sina Mail may have its own proprietary protocols and software, which are not compatible with Apple's infrastructure and operating systems.
3. Sina Mail may not meet Apple's privacy and security standards, and thus, Apple may not consider integrating it into their ecosystem.
4. Apple may have already partnered with other email services that offer similar features as Sina Mail and see no need to invite them.
5. The target market for Apple products and services may not heavily rely on Sina Mail, making it less of a priority to integrate it into their ecosystem.
6. Sina Mail may have its own unique branding and identity, which may not align with Apple's branding strategies.
7. Invitation of Sina Mail may require significant investments in resources and infrastructure, which may not be feasible or practical for Apple at this time.
8. Sina Mail may not have expressed any interest in collaborating or partnering with Apple.
